rtc: define RTC_VL_READ values

Currently, the meaning of the value returned by RTC_VL_READ is undocumented
and left to the driver implementation. In order to get more meaningful
values, define a set of values to use as to make clear to userspace what is
the status of the various voltages feeding the RTC.

Link: https://lore.kernel.org/r/20191214220259.621996-2-alexandre.belloni@bootlin.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

include/uapi/linux/rtc.h

@@ -92,7 +92,12 @@ struct rtc_pll_info {
 #define RTC_PLL_GET	_IOR('p', 0x11, struct rtc_pll_info)  /* Get PLL correction */
 #define RTC_PLL_SET	_IOW('p', 0x12, struct rtc_pll_info)  /* Set PLL correction */
 
-#define RTC_VL_READ	_IOR('p', 0x13, int)	/* Voltage low detector */
+#define RTC_VL_DATA_INVALID	BIT(0) /* Voltage too low, RTC data is invalid */
+#define RTC_VL_BACKUP_LOW	BIT(1) /* Backup voltage is low */
+#define RTC_VL_BACKUP_EMPTY	BIT(2) /* Backup empty or not present */
+#define RTC_VL_ACCURACY_LOW	BIT(3) /* Voltage is low, RTC accuracy is reduced */
+
+#define RTC_VL_READ	_IOR('p', 0x13, unsigned int)	/* Voltage low detection */
 #define RTC_VL_CLR	_IO('p', 0x14)		/* Clear voltage low information */
 
 /* interrupt flags */